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Aviation glossary

Bypass ratio

Also: BPR, bypass

Engines

Ratio between the mass of air that flows around a turbofan's core through the outer duct and the mass that passes through the core to be burnt.

What it is

An engine with a bypass ratio of 10:1 sends ten kilograms of air around the core through the fan duct for every kilogram that passes through the core to be burnt. The higher the figure, the more air is moved at a lower speed, which improves propulsive efficiency, cuts fuel burn and makes the engine quieter.

Early turbofans of the 1960s, such as the Pratt & Whitney JT8D fitted to the Boeing 727 and the DC-9, had ratios of about 1:1. The CFM56 sits between 5 and 6, while current single-aisle engines go further: the LEAP-1A is around 11 and the Pratt & Whitney PW1100G above 12.

There are physical limits. A bigger fan is heavier, adds drag and needs more ground clearance, which helps explain the arrival of the geared turbofan and the renewed interest in open rotors.

On Vuelopedia

CFM International Pratt & Whitney

Related terms

TurbofanJet engine in which a large front fan drives air around the core, producing most of the thrust with greater efficiency and less noise. FanLarge bladed rotor at the front of a turbofan that accelerates the incoming air and generates most of the engine's thrust. Engine coreCentral part of a turbofan, essentially the high-pressure compressor, combustor and high-pressure turbine, through which the air that is burnt passes. Geared turbofanTurbofan with a reduction gearbox between the fan and the turbine that drives it, so that each can turn at its own most efficient speed. Specific fuel consumptionAmount of fuel an engine burns for each unit of thrust or power it produces over a given time; the standard measure of engine efficiency.
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This page was last updated on October 9, 2026

APA

Vuelopedia. (2026, October 9). Bypass ratio. Retrieved October 16, 2026, from https://vuelopedia.org/en/glossary/bypass-ratio

MLA

“Bypass ratio.” Vuelopedia, Fundación Poderia, 9 Oct. 2026, vuelopedia.org/en/glossary/bypass-ratio. Accessed 16 Oct. 2026.

Chicago

Vuelopedia. “Bypass ratio.” Fundación Poderia. Last modified October 9, 2026. https://vuelopedia.org/en/glossary/bypass-ratio.

Harvard

Vuelopedia (2026) Bypass ratio. Available at: https://vuelopedia.org/en/glossary/bypass-ratio (Accessed: 16 October 2026).

BibTeX

@misc{vuelopedia_en_glossary_bypass_ratio,
  author       = {{Vuelopedia}},
  title        = {{Bypass ratio}},
  publisher    = {Fundación Poderia},
  year         = {2026},
  date         = {2026-10-09},
  url          = {https://vuelopedia.org/en/glossary/bypass-ratio},
  howpublished = {\url{https://vuelopedia.org/en/glossary/bypass-ratio}},
  urldate      = {2026-10-16},
  note         = {Accessed 2026-10-16}
}

Wikipedia

{{cite web |url=https://vuelopedia.org/en/glossary/bypass-ratio |title=Bypass ratio |website=Vuelopedia |publisher=Fundación Poderia |date=2026-10-09 |access-date=2026-10-16 |language=en}}

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